fix(ios): make the login WebView identify as Mobile Safari

accounts.spotify.com rejected iOS logins ("Oops, something went wrong") because
the WebView sent WKWebView's default User-Agent, which lacks Safari's
"Version/x ... Safari/604.1" and reads as an embedded browser. The random
User-Agent header never reached any request: the screen replaces the content
before it loads.

Have WebKit build Safari's exact UA (with its frozen OS version) via a probe
WKWebView using Safari's applicationNameForUserAgent, prime it at startup and
apply it through webSettings.customUserAgentString so every request uses it.

composeApp/src/iosMain/kotlin/org/spotube/spotube/MainViewController.kt

@@ -27,6 +27,7 @@ import androidx.compose.ui.window.ComposeUIViewController
 import com.yushosei.newpipe.extractor.NewPipe
 import com.yushosei.newpipe.util.DefaultDownloaderImpl
 import org.spotube.spotube.core.di.initKoin
+import org.spotube.spotube.core.webview.SafariUserAgent
 import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.SupervisorJob
@@ -35,6 +36,7 @@ import org.koin.core.component.inject
 
 fun MainViewController() = ComposeUIViewController(configure = {
     initKoin()
+    SafariUserAgent.prime()
 }) { InitNewPipe { App() } }
 
 @Composable

composeApp/src/iosMain/kotlin/org/spotube/spotube/core/webview/PlatformWebviewConfig.ios.kt

@@ -27,6 +27,8 @@ import platform.WebKit.WKWebsiteDataTypeCookies
 import platform.WebKit.WKWebsiteDataTypeLocalStorage
 
 actual fun platformWebviewConfig(webView: WebViewState, pluginId: String?) {
+    // Applied to the WKWebView itself, so it covers redirects and XHR too, not just the first request.
+    SafariUserAgent.value?.let { webView.webSettings.customUserAgentString = it }
 }
 
 @OptIn(ExperimentalForeignApi::class)

composeApp/src/iosMain/kotlin/org/spotube/spotube/core/webview/SafariUserAgent.kt

@@ -0,0 +1,68 @@
+/*
+ * Copyright (C) 2026 Kingkor Roy Tirtho and Spotube Contributors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package org.spotube.spotube.core.webview
+
+import co.touchlab.kermit.Logger
+import kotlinx.cinterop.ExperimentalForeignApi
+import kotlinx.cinterop.readValue
+import platform.CoreGraphics.CGRectZero
+import platform.UIKit.UIDevice
+import platform.UIKit.UIUserInterfaceIdiomPad
+import platform.WebKit.WKWebView
+import platform.WebKit.WKWebViewConfiguration
+
+/**
+ * Mobile Safari's exact User-Agent on this device.
+ *
+ * WKWebView's default UA ends in `Mobile/15E148` without Safari's `Version/x … Safari/604.1`,
+ * which sites like accounts.spotify.com treat as an embedded browser ("Oops, something went
+ * wrong"). Hand-building the string isn't safe either: Safari freezes the OS part (iOS 26
+ * reports `iPhone OS 18_7`). So WebKit builds it: a probe WKWebView gets Safari's application
+ * name and reports `navigator.userAgent`, which is cached for every web view.
+ */
+internal object SafariUserAgent {
+    var value: String? = null
+        private set
+
+    // Kept alive until its async evaluation calls back.
+    private var probe: WKWebView? = null
+
+    /** Must run on the main thread (WebKit); call once at startup. */
+    @OptIn(ExperimentalForeignApi::class)
+    fun prime() {
+        if (value != null || probe != null) return
+        val config = WKWebViewConfiguration().apply { applicationNameForUserAgent = safariApplicationName() }
+        probe = WKWebView(frame = CGRectZero.readValue(), configuration = config).also { webView ->
+            webView.evaluateJavaScript("navigator.userAgent") { result, _ ->
+                value = result as? String
+                probe = null
+                Logger.withTag("SafariUserAgent").i { "WebView User-Agent: $value" }
+            }
+        }
+    }
+
+    // What Safari appends after WebKit's base UA; iPad Safari sends the desktop-class form.
+    private fun safariApplicationName(): String {
+        val version = UIDevice.currentDevice.systemVersion.split('.').take(2).joinToString(".")
+        return if (UIDevice.currentDevice.userInterfaceIdiom == UIUserInterfaceIdiomPad) {
+            "Version/$version Safari/605.1.15"
+        } else {
+            "Version/$version Mobile/15E148 Safari/604.1"
+        }
+    }
+}